Stress evolution of restrained concrete is a significant direct index in early-age cracking (EAC) analysis of concrete. This study presents experiments and numerical modelling of the early-age stress evolution of Ground granulated blast furnace slag (GGBFS) concrete, considering the development of autogenous deformation and creep. Temperature Stress Testing Machine (TSTM) tests were conducted to obtain the autogenous deformation and stress evolution of restrained GGBFS concrete. By a self-defined material subroutine based on the Rate-type creep law, the FEM model for simulating the stress evolution in TSTM tests was established. By characterizing the creep compliance function with a 13-units continuous Kelvin chain, forward modelling was firstly conducted to predict the stress development. Then inverse modelling was conducted by Bayesian Optimization to efficiently modify the arbitrary assumption of the codes on the aging creep. The major findings of this study are as follows: 1) the high autogenous expansion of GGBFS induces compressive stress at first hours, but its value is low because of high relaxation and low elastic modulus; 2) The codes highly underestimated the early-age creep of GGBFS concrete. They performed well in prediction of stress after 200 h, but showed significant gaps in predictions of early-age stress evolution; 3) The proposed inverse modelling method with Bayesian Optimization can efficiently adjusted the aging terms which produced best modelling results. The adjusted creep compliance function of GGBFS showed a much faster aging speed at early ages than the one proposed by original codes.@en

Tailoring lattice structures is a commonly used method to develop lattice materials with desired mechanical properties. However, for cementitious lattice materials, besides the macroscopic lattice structure, the multi-phase microstructure of cement paste may have substantial impact on the mechanical responses. Therefore, this work proposes a multi-scale numerical modelling method to simulate the deformation and fracture behavior of cementitious lattice materials, such that the influence of cement paste microstructure can be properly captured. On the microscale, the load–displacement response of cement paste is numerically simulated then experimentally validated. In order to rationally investigate the role of cement paste microstructure, the obtained load–displacement response was then formulated to several types of model inputs reflecting different degree of brittleness. These inputs were then used for simulating the mechanical response of macroscale cementitious lattices. By comparing the simulation to experiment, multi-linear behavior (ML) was found to an appropriate method to include the realistic pre-critical cracking and post-peak softening of cement paste in the model. Compared to ideally brittle behavior, using ML as input, the discrepancy between simulated and experimentally tested fracture energy decreases from 37.4% to 12.4%. In addition, the influence of lattice structure on the strength of cementitious lattices was also accurately captured by the proposed model. Randomized cementitious lattice has 21.6% (22.0% from simulation) lower strength than regular lattice. Moreover, the influence of fracture criterion of the proposed model is discussed and elaborated. Owning to the high simulation accuracy, the proposed multi-scale method in this work could be helpful for tailoring the fracture cementitious lattice materials for future studies.@en

For a single batch material, time intervals and nozzle standoff distances between two subsequent layers are two critical printing parameters that influence the mechanical performance of the printed concrete. This paper presents an experimental and numerical study to investigate the impacts of these printing parameters on the interlayer bond strength of the 3D printed limestone and calcined clay-based cementitious materials. All samples were manufactured by a lab-scale 3D printer equipped with a hybrid back- and down-flow nozzle (rectangular opening). The uniaxial tensile test was employed to quantify the interface adhesion of printed specimens. Moreover, the greyscale value image of microstructure, as well as the air void content and distribution of the printed specimens were acquired by X-ray computed tomography and characterized by image analysis. The experimental results showed that extending the time interval between construction of two layers could decrease the bond strength, whereas only increasing the nozzle standoff distance exhibited limited effects on that. The weak bond strength could be attributed to the high local porosity at the interface of the specimen. Additionally, numerical simulations of the uniaxial tensile test were conducted using a 2D lattice fracture model, which can predict the bond strength of printed specimens for different void content in the interface layer.@en

Cementitious materials are widely used in construction. For their low ductility, they typically need to be reinforced by steel rebars, which cause potential corrosion problems. Polymeric reinforcement, which does not have corrosion problems, has been used to replace steel rebars. However, a relatively high reinforcing ratio is usually required for the cementitious composites reinforced by conventional polymeric reinforcement. Owing to the customizability of 3D printing technology, polymeric reinforcement with a functionally graded structure is able to be manufactured, which significantly reduces the reinforcing ratio of the reinforced cementitious composites meanwhile improves their mechanical properties. In this present study, 3D printed polymeric octet lattice structures were used as reinforcement to develop cementitious composites with enhanced ductility. Four-point bending experiments were performed on the plain mortar, and the reinforced specimens and a finite element model was used to simulate the experiments numerically. A good agreement between experiments and simulations was found: the reinforced specimens have a significantly increased flexural ductility comparing to plain mortar. Composites reinforced by vertically functionally graded lattice structures have a significantly lower reinforcing ratio while exhibiting obviously higher normalized ductility. In addition, the fracture behavior of the reinforced cementitious composites was evaluated using a fracture energy based analytical model. The analysis shows that, from the perspective of fracture energy release, the steady state cracking criteria were not satisfied by the cementitious composites developed in this study so that multiple cracking and strain hardening behavior was not obtained. However, according to numerical predictions, increasing strength of the printed reinforcement material by 40% would allow these behaviors to be potentially achieved. This work shows that additive manufacturing has great potential for developing reinforcement for cementitious materials to reduce the reinforcing ratio and enhance ductility.@en

Application of micromechanical modelling of hydrated cement paste (HCP) gains more and more interests in the field of cementitious materials. One of the most promising approaches is the use of so-called microstructure informed micromechanical models, which provides a direct link between microstructure and mechanical properties. In order to properly model the micromechanical properties of HCP, advanced mechanical models, well-characterised microstructures and proper input parameters are required. However, due to the complex material structure of HCP, this is not an easy to achieve for any of the three aforementioned aspects. Therefore, this paper aims at reviewing of the techniques that have been developed to contribute to the micromechanical modelling. Basic principles, corresponding research results, recent advances and limitations are given. It is expected that this review can help researchers make reasonable choices on techniques for the micromechanical modelling of cementitious materials.@en

This paper aims to provide a systematical review of the available printing strategies, sustainable cementitious materials and characterization methods for extrusion-based 3D concrete printing (3DCP). The printing strategies, consisting of printing setup, process, and material requirements, were summarized initially. In the material aspect, the high ordinary Portland cement (OPC) content in most printable mixtures is a major issue that impedes the sustainability of 3DCP. This can be resolved by partially substituting OPC with supplementary cementitious materials (SCMs). In this review, the effect of different SCMs on fresh-state behaviors and 3D printing of cementitious materials was comprehensively discussed. Finally, a series of test methods for quantitively characterizing fresh properties, 3D printability and interlayer behaviors were summarized and reviewed.@en

This paper reports the carbonation characteristics of a cement-slag system exposed to accelerated carbonation testing, and its improved carbonation resistance with the increasing MgO content in blast furnace slag, in which hydrotalcite-like phase plays a key role. Our research showed that the hydrotalcite-like phase started to carbonate upon contacting with the carbonate ions and bound more than 15 wt% CO2−3 in the mildly carbonated and transition areas. This value was positively associated with the magnesia content of slag. Additionally, the proportion shared by hydrotalcite-like phase decreased in the fully carbonated area, and more CO2 was fixed in the form of calcium carbonate. Consistent with the thermodynamic modelling, the ratio of CO2 bound in carbonated hydrotalcite-like phase to the total CaCO3 continued to decrease as the CO2 ingress progressed. On the other hand, the reaction between hydrotalcite-like phase and CO2 was found to be volumetrically stable due to binding CO2 in the interlayer space, and Mg was still distributed within the original slag grain region. Mg/Al atomic ratio of hydrotalcite-like phase remained nearly the same before and after carbonation. Results of this study quantitatively emphasized the favorable effect of hydrotalcite-like phase to improve the carbonation resistance of slag-rich cementitious systems.@en

Carbonation of hydrated cement paste (HCP) causes numerous chemo-mechanical changes in the microstructure, e.g., porosity, strength, elastic modulus, and permeability, which have a significant influence on the durability of concrete structures. Due to its complexity, much is still not understood about the process of carbonation of HCP. The current study aims to reveal the changes in porosity and micromechanical properties caused by carbonation using micro-beam specimens with a cross-section of 500 μm x 500 μm. X-ray computed tomography and micro-beam bending tests were performed on both noncarbonated and carbonated HCP micro-beams for porosity characterization and micromechanical property measurements, respectively. The experimental results show that the carbonation decreases the total porosity and increases micromechanical properties of the HCP micro-beams under the accelerated carbonation. The correlation study revealed that both the flexural strength and elastic modulus increase linearly with decreasing porosity.@en

The aim of this work is to investigate the mechanical performance of hardened cement paste (HCP) under compression at the micrometre length scale. In order to achieve this, both experimental and numerical approaches were applied. In the experimental part, micrometre sized HCP specimens were fabricated and subjected to uniaxial compression by a flat end tip using nanoindenter. During the test, the load-displacement curves can be obtained. In the modelling part, virtual micrometre sized specimens were created from digital material structures obtained by X-ray computed tomography. A computational compression test was then performed on these virtual specimens by a discrete lattice fracture model using the local mechanical properties calibrated in the authors' previous work. A good agreement is found between the experimental and numerical results. The approach proposed in this work forms a general framework for testing and modelling the compression behaviour of cementitious material at the micrometre length scale.@en

This study presents an experimental investigation of the rate-dependent mechanical properties of cement paste at the microscale. With the use of a nanoindenter, micro-cantilever beams with the size of 300 μm × 300 μm × 1650 μm were loaded at five different strain rates from around 10−6/s to 10−2/s until failure. It is found that with increasing strain rate, the stress-strain curves show less and delayed pre-peak nonlinearity. Both the flexural strength and the elastic modulus of beams increase with increasing strain rate, while the strain at peak stress exhibits an opposite trend. Examination of the fracture surface indicates that with increasing strain rate the possibility of a crack to pass through stronger components of the hydration products is increased. The experimental observations and possible mechanisms leading to changes in mechanical responses are discussed. It is suggested that at least two micromechanical processes, namely creep and Stéfan effect, are mainly responsible for the rate-dependent behaviour of cement paste within the investigated strain rate range and their dominances seem to vary with the strain rate. At lower strain rate, the strain rate sensitivity of cement paste is thought to be dominated by the creep effect, while at higher strain rate the Stéfan effect appears to be the governing factor.@en

This study aims to provide an efficient alternative for predicting creep modulus of cement paste based on Deep Convolutional Neural Network (DCNN). First, a microscale lattice model for short-term creep is adopted to build a database that contains 18,920 samples. Then, 3 DCNNs with different consecutive convolutional layers are built to learn from the database. Finally, the performance of DCNNs is tested on unseen testing samples. The results show that the DCNNs can achieve high accuracy in the testing set, with the R2 all higher than 0.96. The distribution of creep modulus predicted by the DCNNs coincides with that of the original data. Furthermore, through analyzing the feature maps, it is found that the DCNNs can correctly capture the local importance of different microstructural phases. The DCNN allows therefore prediction of the creep modulus based on microstructural input, which saves computational resources of segmentation procedure and multiple incremental FEM calculations.@en

In this study, a numerical model using a 2D lattice network is developed to investigate the fatigue behaviour of cement paste at the microscale. Images of 2D microstructures of cement pastes obtained from XCT tests are used as inputs and mapped to the lattice model. Different local mechanical and fatigue properties are assigned to different phases of the cement paste. A cyclic constitutive law is proposed for considering the fatigue damage evolution. Fatigue experiments performed at the same length scale are used to calibrate and validate the model. The proposed model can reproduce well the flexural fatigue experimental results, in terms of S-N curve, stiffness degradation and residual deformation. The validated model is then used to predict the uniaxial tensile fatigue fracture of cement paste. The effects of microstructure and stress level on the fatigue fracture are studied using the proposed model. This model forms a basis for the multiscale analysis of concrete fatigue.@en

This study aims to provide an efficient and accurate machine learning (ML) approach for predicting the creep behavior of concrete. Three ensemble machine learning (EML) models are selected in this study: Random Forest (RF), Extreme Gradient Boosting Machine (XGBoost) and Light Gradient Boosting Machine (LGBM). Firstly, the creep data in Northwestern University (NU) database is preprocessed by a prebuilt XGBoost model and then split into a training set and a testing set. Then, by Bayesian Optimization and 5-fold cross validation, the 3 EML models are tuned to achieve high accuracy (R2 = 0.953, 0.947 and 0.946 for LGBM, XGBoost and RF, respectively). In the testing set, the EML models show significantly higher accuracy than the equation proposed by the fib Model Code 2010 (R2 = 0.377). Finally, the SHapley Additive exPlanations (SHAP), based on the cooperative game theories, are calculated to interpretate the predictions of the EML model. Five most influential parameters for concrete creep compliance are identified by the SHAP values of EML models as follows: time since loading, compressive strength, age when loads are applied, relative humidity during the test and temperature during the test. The patterns captured by the three EML models are consistent with theoretical understanding of factors that influence concrete creep, which proves that the proposed EML models show reasonable predictions.@en

This study presents an experimental investigation of fatigue properties of cement paste at the microscale. A strong size dependence is found for the flexural fatigue life of the cement paste specimen. Microscopic observations on the fractured surfaces suggest that there is a higher density of nano-scale cracks generated during the fatigue loading compared to the static fracture. However, the fatigue damage evolution is found to be very slow and small even under high stress levels. The development of residual deformation for cement paste can be explained by the combined effects of viscoelastic deformation and fatigue cracking growth.@en

In this study, the flexural strength and fatigue properties of interfacial transition zone (ITZ) were experimentally investigated at the micrometre length scale. The hardened cement paste cantilevers (150 × 150 × 750 μm3) attached to a quartzite aggregate surface were prepared and tested under the monotonic and cyclic load using a nanoindenter. The measured flexural strength of the ITZ (10.49–14.15 MPa) is found to be one order of magnitude higher than the macroscopic strength of ITZ reported in literature. On the other hand, the fatigue strength of the ITZ is lower than that of bulk cement paste at same length scale, measured previously by the authors. The microscopic mechanical interlocking and the electrostatic interaction between aggregate surface and hydration products are thought to contribute to the bond strength of ITZ. This study provides an experimental basis for the development of multiscale analysis of concrete subjected to both static and fatigue loading.@en

This study proposes an experimental method for studying the short-term creep behaviour of cement paste at micro-scale. The micro-bending tests on miniaturized cantilever beams were used to characterize the viscoelastic properties of cement paste. The effects of w/b ratio, the type of binder and the stress level on the microscopic creep behaviour were investigated. It is found that the short-term creep of cement paste at microscale can be satisfactorily described by a power-law function. A linear viscoelastic behaviour has been observed in different cementitious systems at the microscale with the stress level up to 67.9%. When compared with the creep results in microindentation tests and conventional macroscopic tests, the obtained creep compliance function in this study is found to be both qualitatively and quantitatively representative of the macroscopic results. This experimental study underlines the importance of microstructural effect on the creep behaviours of cementitious materials at microscale.@en

This paper presents an experimental investigation on the short-term creep recovery of cement paste at micrometre length scale. Micro-cantilever beams were fabricated and tested with 8 different loading series using the nanoindenter. It is found that cement pastes show high recovery ratios (>80%) even subjected to very high stress levels. Relatively lower recovery ratios and non-linear creep were also observed for w/c 0.4 samples under high stress levels. A good agreement is found between the results predicted using the linear superposition principle and the experimental results except for the measured non-linear creep in w/c 0.4 samples. It is suggested that the short-term creep recovery may be associated with the microscale stress redistribution or the reversible internal water movement. The observed non-linear creep under the highest stress level may be due to the higher density of microcracks generated during the loading stage, which may further promote the water transfer.@en

Cement paste is the main binding component in concrete and thus its fundamental properties are of great significance for understanding the fracture behaviour as well as the ageing process of concrete. One major aim of this paper is to characterize the micromechanical properties of cement paste with the aid of a nanoindenter. Besides, this paper also presents a preliminary study on the fatigue behaviour of cement paste at the micrometer level. Miniaturized cantilever beams made of cement paste with different water/cement ratios were statically and cyclically loaded. The micromechanical properties of cement paste were determined based on the measured load-displacement curves. The evolution of fatigue damage was evaluated in terms of the residual displacement, strength, and elastic modulus. The results show that the developed test procedure in this work is able to produce reliable micromechanical properties of cement paste. In addition, little damage was observed in the cantilever beams under the applied stress level of 50% to 70% for 1000 loading cycles. This work may shed some light on studying the fatigue behaviour of concrete in a multiscale manner.@en

Slag rim mainly consists of secondary precipitations such as C–S–H gel phase and hydrotalcite-like phase, which originate from the hydration of slag. In this paper, the micro-mechanical properties of slag rim were characterized by nanoindentation in combination with SEM. It was found that, compared to the C–S–H gel phase, slag rim showed about a 15 GPa higher modulus of elasticity. At the early age, slag cement paste mainly consisted of low-density C–S–H gel phase, high-density C–S–H gel phase, and unhydrated slag particles, as well as calcium hydroxide based on the results of nanoindentation; at the later age, the system contained C–S–H gel phase, slag rim (a mixture of C–S–H gel phase and hydrotalcite-like phase), and unhydrated slag particles.@en

Ageing is an inherent feature of cementitious materials. Ageing of material leads to gradual loss of the function of a concrete structure with increasing likelihood of failure. As one of the typical ageing phenomena, fatigue of concrete, has recently received considerable research attention. The phenomenon of concrete fatigue is complicated as it inherently involves multiple spatial scales owing to the multiscale heterogeneous nature of concrete. Over the past century, tremendous efforts have been devoted to concrete fatigue. However, many important scientific problems still remain unsolved. These problems include at least: how does the multiscale heterogeneous material structure of concrete affect the fatigue behaviour; how does fatigue damage evolve inside the concrete; how to properly simulate and predict the fatigue behaviour. The main o bjective of this thesis is to improve the knowledge of fatigue behaviour of cementitious materials at the microscale and to develop a multiscale modelling scheme from micro- to meso-scale to estimate the fatigue properties. Firstly, experimental techniques for characterization of mechanica! and fatigue properties of cementitious materials at the microscale are developed. These techniques include the preparation and testing of micrometre sized sample. For sample preparation, a precision micro-dicing machine is used. With the help of a nanoindentation measurement device, micro-bending tests are performed to study the flexural fatigue behaviour of two major components in the mortar, i.e. the cement paste and the interfacial transition zone (ITZ). The fatigue fracture surface and damage evolution are assessed using an Environmental Scanning Electron Microscope (ESEM) and X-ray computed tomography (XCT). The mechanica! properties, including the strength and elastic modulus, as well as the fatigue properties including the relationship between fatigue life and stress level, the stiffness degradation and residual deformation evolution, can be obtained using the developed microscale testing approach. Secondly, a numerical model using a 2D lattice network is developed to simulate the fatigue behaviour of cementitious material at the microscale. Images of 2D microstructures of cement pastes and ITZ obtained from XCT tests are used as inputs and mapped to the lattice model. Different local mechanica! and fatigue properties are assigned to different phases of the cement paste and interfacial transition zone. A constitutive law for cyclic loading is proposed to consider the fatigue damage evolution. Experimental results obtained at the microscale are used to calibrate and validate the model. The developed model is able to investigate the effect of microstructure heterogeneities on fatigue damage evolution in a very efficient way and can establish a quantitative relationship between the material structure and the global fatigue performance. Moreover, it can also provide valuable insight into the fatigue damage evolution and fatigue fracture phenomena under different stress levels. Finally, a parameter-passing scheme is adopted for upscaling of the mechanica! and fatigue properties. Via this approach, the global fatigue fracture behaviour (i.e. stress-strain response and S-N relation) simulated at smaller scale can be used as input for the fatigue fracture modelling at the larger scale. The model can satisfactorily predict crack patterns, mechanica! and fatigue properties under fatigue loading. The model has fully predictive capabilities at the mesoscale. Hence, the model offers an opportunity to investigate in more detail the influence of material structures at different scales on the macroscopie fatigue performance. @en